A. Field of the Invention
The present disclosure generally relates to positive displacement rotary devices. The disclosed embodiments relate more specifically to positive displacement rotary devices for generating power at an output shaft and methods for making same.
B. Related Technology
In general, conventional gas turbines have three basic stages 1) compression, 2) combustion, and 3) a power extraction. Energy extracted from a turbine is used to drive a compressor, which compresses air so that it may be mixed with fuel and burned in the combustor. The burnt fuel then exits the combustor through the turbine, which causes the turbine to rotate. The rotation of the turbine drives both the compressor and an output shaft.
Different types of gas turbines are defined by how much energy is extracted from the output shaft. For example, turbojets extract as little energy as possible from the output shaft to drive one or more compressor stages, such that much of the energy may be extracted as jet thrust from the compressed gases exiting the turbine. By contrast, turboshafts extract as much energy as possible from the output shaft to not only drive one or more compressor stages, but also to drive other machinery.
Gas turbines are dynamic devices, rather than positive displacement devices. In other words, the output shaft of a gas turbine moves in reaction to the pressure generated when fluid moving at a high speed is diffused, or slowed down, with the blades of the compressor and the turbine, rather than in reaction to pressure differences created on opposing sides of those blades in a constant volume of fluid. And while positive displacement devices move a nearly fixed volume of fluid per revolution of the output shaft at all speeds, the volume of air that a gas turbine moves must increase with the square of the revolutions of the output shaft. Accordingly, gas turbines are efficient at operating speeds that are well below their design speeds. Paradoxically, those operating speeds are often above a speed that is practical to directly drive other machinery with the output shaft, such that more complicated machinery (e.g., a reduction gear) must be implemented to interface the output shaft of a gas turbine with other machinery.
In operation, gas turbines may be started by driving them with a starter motor. For example, the gas turbine may be driven to a speed where the compressor provides enough air pressure for fuel to be ignited in a combustor. If that speed is too great, however, the turbine may begin to act as a positive displacement fixed vane compressor, which would create a vacuum in the combustor. Combustion requires oxygen to react with fuel, and the greater the vacuum created in the combustor, the fewer oxygen molecules there are that may react with the fuel. Another problem with reduced pressure in the combustor is that compressed gas is hotter than ambient air, while the decompressed air in a vacuum is cooler. Such cooled air provides a worse environment for combustion. The possibility of creating such conditions further limits the operating speed of gas turbines.
Positive displacement devices also have various limitations. For example, internal combustion engines configured as positive displacement devices (e.g. piston engines, Wankle engines, etc.) historically have not provided combustion in a constant volume. Instead, such reciprocating machines confine the charge gas, reduce its volume in a compression cycle, and then extract energy from an output shaft as the volume of the charge gas increases after being combusted in an expansion cycle. That process is highly inefficient due to losses not only from the compression cycle, but also from decreases in temperature during the expansion cycle.
In an effort to increase the power density of the reciprocating engine, hybrids of positive displacement devices and gas turbines have been developed. In a turbocharged reciprocating engine, for example, the reciprocating engine serves as the combustor for the turbine and the only work the turbine does is to drive the compressor that increases the air flow to the reciprocating engine so that it can burn more fuel. And in a supercharged reciprocating engine, the reciprocating engine drives a compressor with shaft power, rather than indirectly with combustion gases and a turbine. Nevertheless, many controls are required to effectively mate a dynamic compressor to a positive displacement device, such as the use of waste gates on turbochargers. Further, the limited operating speeds of dynamic compressors generally prevents their use when they are driven by the output shaft of the reciprocating engine, such as in supercharged reciprocating engines. Instead, less efficient positive displacement compressors generally are used in such applications.